Pluto's Heavy Heart, Not a Massive Impact, Caused Its Depression

Two weeks after scientists with NASA's New Horizons mission published research showing that Pluto may have an underground ocean, another team has an alternative explanation for how the planet's distinctive heart-shaped basin formed.

The new paper, published Wednesday in Nature, suggests the basin got its start not from a crashing comet or other impacting body, but from the weight of ices that collected on Pluto's surface, an explanation that doesn't require the existence of an ocean.

"There is an ocean-free path to explaining the key features of Sputnik Planitia," University of Maryland astronomer Douglas Hamilton wrote in an email to Seeker.

"But while Pluto need not have an interior ocean, conversely, nothing in my study argues against such an ocean," Hamilton added.

Computer models show that Pluto's ice, similar to the Greenland Ice Sheet, may have acted alone to form a basin by pushing down on the underlying crust.

"Thinking that the basin formed from the weight of the ices really came late in the creative process," Hamilton said. "My initial focus was to explain the location of the ice cap - the ices of Sputnik Planitia - on the surface of Pluto. The feature is centered at 25 north latitude and 175 longitude, nearly opposite the location of the giant moon Charon.

"In coming up with a viable scenario to explain these observations I realized that starting with an impact limits successful models to just a handful of impact locations. While that remains possible, a more robust model would explain the ice cap's location from any starting conditions," he wrote.

"My model does just that by not relying on an impact. But one still needs to explain why all of these ices are found in a deep basin. I argue that the basin forms simply from the immense weight of the massive ice cap - the planet's crust sags a bit under the weight, just as has happened for Greenland, Canada, and Scandinavia during the last ice age. This is a natural explanation for the coincident locations of the ice cap and the basin," Hamilton said.

The new study theorizes that the ice cap formed early, when Pluto was still spinning quickly, and that the basin formed later. The ice cap creates a slight asymmetry that either locks toward or away from Charon when Pluto's spin slows to match the orbital motion of the moon, Hamilton added.

The unusual location of the ice cap is due to Pluto's climate and spin axis, which is tiled by 120 degrees (compared to Earth's tile of 23.5 degrees), the University of Maryland said in a related press release.

"Modeling the dwarf planet's temperatures showed that when averaged over Pluto's 248-year orbit, the 30 degrees north and south latitudes emerged as the coldest places on the dwarf planet, far colder than either pole. Ice would have naturally formed around these latitudes, including at the center of Sputnik Planitia," the press release said.

Over time, the ice deposit attracted more ice by reflecting away solar light and heat, keeping temperatures low, a phenomenon known as a "runaway albedo effect."

Because Pluto's basin is bigger than the volume of ice that fills it today, scientists conclude that Sputnik Planitia has been losing mass over time.

Pluto is only the third body in the solar system, along with Earth and Mars, that has ice caps.